Silicon carbide, a crystalline compound of silicon and carbon, has long been known for its hardness, its strength and its excellent resistance to oxidation and corrosion. Silicon carbide has a low coefficient of expansion, good heat transfer properties, and maintains high strength at elevated temperatures. In recent years, the art of producing high density silicon carbide bodies from silicon carbide powders has been developed. Methods include reaction bonding, chemical vapor deposition, hot pressing and pressureless sintering (initially forming the article and subsequently sintering). Examples of these methods are described in U.S. Pat. Nos. 3,853,566; 3,852,099; 3,954,483; and 3,960,577. The high density silicon carbide bodies so produced are excellent engineering materials and find utility in fabrication of components for turbines, heat exchange units, pumps and other equipment or tools that are exposed to severe wear and/or operation under high temperature conditions. The present invention relates methods of producing silicon carbide articles that have high-density and high-strength characteristics.
In order to obtain high-density and high-strength silicon carbide ceramic materials, various densification additives have been utilized. For example, a method of hot pressing silicon carbide to densities in the order of 98 percent of theoretical by addition of aluminum and iron as densification aids is disclosed by Alliegro, et al., J. Ceram. Soc., Vol. 39, No. 11, Nov., 1956, pages 386 to 389. They found that a dense silicon carbide could be produced from a powder mixture containing 1 percent by weight of aluminum. Their product had a modulus of rupture of 54,000 psi. at room temperature and 70,000 psi. at 1371.degree. C. A more recent advance is the use of boron as a densification aid. Usually, such aids are added in amounts in the range between about 0.3 and about 3.0 percent by weight of the powder mixture to be sintered. The boron additive may be in the form of elemental boron or in the form of boron-containing compounds, for example, boron carbide. Examples of boron-containing silicon carbide powders may be found in U.S. Pat. Nos. 3,852,099; 3,954,483; and 3,968,194.
The presence of excess amounts of boron, greater than that required to insure a dense sintered product, may act to lower the inherent strength of the sintered product, thereby limiting its uses. Also, the presence of excess boron in the sintered product may, in some instances, be detrimental to the oxidation resistance of the product. For these reasons, lessening the amount of boron or boron-containing compounds added to the initial silicon carbide powder mixture is advantageous. However, up to now, the desired density of the sintered silicon carbide product, greater than 90 percent of theoretical, and, preferably, greater than 95 percent of theoretical, has not been obtainable in pressureless sintering processes except by the addition of densification aids to the initial starting material.